The proposed research is a study of electron and proton transfer reactions that are important for energy conversion processes in membrane proteins. The research is focused on the reaction of the bound ubiquinone QB in the photosynthetic reaction center (RC) from the bacterium Rhodobacter sphaeroides. 2e -+2H+ +QB -->QBH2. Site directed mutagenesis will be used to investigate the mechanism of proton-coupled electron transfer and the pathway and mechanism for proton transfer. The electron transfer rates for these reactions are complex and are believed to involve protein dynamics as well as proton transfer. The molecular basis for the observed electron transfer rate will be examined by site directed mutagenesis to elucidate the factors that control the electron transfer rate. The proton transfer pathway into the QB site is believed to involve surface His residues and a chain of acidic residues and water molecules. The factors important for proton transfer will be investigated by modification of the environment of the residues in the proton transfer pathway and measurement of proton transfer rates using methods that we have developed. The structures of the mutants with modified proton and electron transfer rates will be examined by x-ray crystallography, and spectroscopic techniques including electron paramagnetic resonance, electron nuclear double resonance and infrared spectroscopy. The proposed research would help elucidate the molecular basis for electron and proton transfer mechanisms found in many biological systems. The combination of functional assays, genetic information and structural biology provide an understanding of the molecular basis of these important biological processes and should help lay the foundations for future advances in molecular medicine.